000
FXAK68 PAFC 121349
AFDAFC
Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
449 AM AKST Tue Dec 12 2017
.ANALYSIS AND UPPER LEVELS...
The southern mainland remains entrenched in a deep southerly flow
pattern emanating from the subtropical Pacific between a stout,
persistent ridge over western North America and a broad longwave
trough extending from the Russian Arctic through the Bering Sea
and well into the North Pacific. The next in the parade of
embedded systems tracking northward through the Gulf is bearing
down on the Gulf coast this morning as a front moves through the
northern Gulf and the parent 978 mb low nears the Alaska
Peninsula. Scattered rain showers continue along the coast and
over Southwest AK ahead of these features, with interior portions
of Southcentral staying mostly dry in a relative lull between
systems. The western half of the Bering remains cold enough for
snow in northerly flow on the back side of the trough, with
widespread snow showers and gusty north winds impacting the
Pribilof Islands and the central part of the Aleutian chain.
&&
.MODEL DISCUSSION...
Models remain in good synoptic agreement as the western CONUSridge holds strong through mid to late week, but they continue
to differ on some of the details of the various disturbances
moving north towards the mainland. There is good agreement on the
current system approaching the Akpen, but solutions then diverge
rather quickly with the next wave coming up Tuesday night into
Wednesday. Utilized a NAM/GFS blend for the morning package to
depict a middle ground approach for these features until small
scale details are better resolved. Generally speaking, the active,
warm, and wet pattern will continue through the short term, with
the bulk of the precipitation focused along the coast and inland
locations remaining mostly downsloped. Guidance is then hinting at
the ridge finally breaking down later in the week, which is
covered in greater detail in the Long Term portion of the
discussion.
&&
.AVIATION...
PANC...Low level wind shear will redevelop this morning as a front
pushes inland along the Gulf coast and winds remain light and
northerly at the surface. Southeast winds will move into the
terminal in the afternoon off of Turnagain Arm as the down inlet
pressure gradient weakens. Winds will then shift back to the north
around 08z Wednesday as the down inlet gradient is reestablished,
with another period of wind shear through Wednesday morning.
Persistent downslope flow is expected to keep the terminal at VFR
conditions through the end of the TAF period.
&&
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The upper levelridge over northwest Canada will remain in place
through Wednesday night, with a trough over the eastern Bering
Sea. This will bring continued southerly flow aloft to the area,
and will keep temperatures quite warm. A weather front along the
north Gulf coast will move inland today. Most of the precipitation
with this system will fall along and near near the coast, and
mostly as rain for the lower elevations. The next system will push
into the Gulf later today, and reach the coast tonight. This
system will have a strong pressure gradient, and fairly strong
winds will are expected across the Gulf, as well as through
Turnagain Arm. This system will move inland Wednesday night, with
winds and precipitation diminishing.
&&
.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2: Tuesday and
Wednesday)...
A strong area of low pressure is making its way northward towards
the Alaska Peninsula this morning. Ahead of the low, gusty
southeast winds are causing most of the precipitation to dry out
as it downslopes off the Aleutian and Alaska Ranges, keeping areas
immediately west of those ranges largely dry. Further west, the
opposite upsloping is locally enhancing the precipitation on the
east side of the Kuskokwim Mountains, with snow showers aided by
upper level forcing continuing past the mountains into the
Kuskokwim Delta area, primarily from Bethel westward. The low will
cross the Alaska Peninsula into western Bristol Bay later this
morning, then continue northward, tracking along the coast. That
will continue the southeast winds through Bristol Bay and the
Lower Kuskokwim Valley, with minimal precipitation expected today
there. However, from Dillingham north and west, upsloping will
enhance the precipitation throughout the day.
As the first low tracks into the Bering Strait tonight,
precipitation will continue along the Kuskokwim Mountains and
redevelop in parts of the Bristol Bay and Lower Kuskokwim Valley
in association with the next low, which by Wednesday morning will
be centered between Kodiak Island and the Alaska Peninsula. It
will bring another slug of warm air, so most populated lower-
elevation areas will see mainly rain as temperatures peak in the
upper 30s to mid 40s. The low will continue through interior
Southwest Alaska during the day Wednesday, which will turn the
flow largely southwesterly across the region, keeping moisture
flowing into the area from the Bering. Meanwhile atmospheric
temperatures will gradually cool behind the low, beginning a
turnover of the lingering precipitation back over to snow from
west to east Wednesday and Wednesday night. Temperatures continue
to cool back towards seasonal normals going into Thursday.
&&
.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
As the low approaching the Alaska Peninsula moves up along the
coast of mainland Alaska today, the persistent northerly winds and
snow showers that have been ongoing across the central Bering
will slowly diminish and shift eastward going into tonight. The
next low will track a good 200 miles further east across interior
Southwest Alaska today, bringing those northerly winds and much
cooler mid-atmospheric air into the coast by Wednesday afternoon.
Meanwhile a small area of high pressure builds eastward as well
through the central Bering and Eastern Aleutians by Wednesday
afternoon, temporarily diminishing the precipitation in the area,
but doing relatively little to thin out the clouds. The weak high
will remain over the eastern Bering Wednesday night.
Across the western Bering, a strong front bringing warmer air into
the area will move into the western Aleutians late this afternoon
and this evening, bringing widespread snow showers to the area
before the warmer air builds in and causes the precipitation to at
least mix with rain. A series of triple-point lows developing
along the front will help advance the warmer air and associated
precipitation further east with time into the central Bering and
Aleutians on Wednesday and Wednesday night. Unsettled weather will
continue throughout the western half of the Bering through
Wednesday night and beyond.
&&
.LONG TERM FORECAST (Days 3 through 7)...
An active pattern remains through the long range forecast as
multiple low pressure systems track across the southern mainland
from the Gulf and Bering. Beginning Wednesday night, the
persistent ridge pattern remains amplified over the Southeast with
a new low moving east across the Southern Bering along a zonal
jet that sets up south of the Aleutians by mid week. This system
is projected to break down the ridge anchored over the Southeast
heading into the weekend. However, there is high uncertainty on
how quickly the ridge breaks down as models continue to diverge in
the low track as it moves to the eastern Bering on Friday.
The storm system rides north of the jet streak and therefore is
not on a fast track. Current model trends are now showing the
streak running ahead of this system, amplifying a new ridge along
the southeast during the weekend. This projection shunts eastward
progress of this low with all models struggling with the strong
shortwaves rotating around the periphery of its center. The ECMWF
and its ensembles had best run to run consistency and was heavily
weighted in the WPC blends which used a blended approach of all
operational models and ensembles.
&&
.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...GaleWarning 119 120 125 130>132 150 178.
&&
$$
SYNOPSIS AND MODEL DISCUSSION...CB
SOUTHCENTRAL ALASKA...BL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JW
LONG TERM...KH/KVP